Process for electrolyzing solutions containing tin



Aug. 26, 1941. D. L. OGDEN min. I

PROCESS FOR ELECTROLYZING SOLUTIONS CONTAINING TIN 2 Sheets-Sheet 1Filed Aug. 6, 193'? BEELE/N N kw N N M N wwwmvw W N. XEVR Qvfi Aug. 26,1941. D. OGDEN EIAL PROCESS FOR ELECTROLYZING SOLUTIONS CONTAINING TFiled Aug. 6, 1937 2 Sheets-Sheet 2 INVENTORS DANIEL L- OGDEN MAX E W.HEBERLE/N 1% 01k ATTORZ ,6 EYS lyzing solutions ratus therefor.

,aqueous solutions containing Patented Aug. 26, 1941 PROCESS FORELECTROLYZING SOLUTIONS CONTAINING TIN Daniel Lattimer Ogden,Woodbridge, and Max F.

W. Heberlein, Rahway, N. American Metal Company,

1., assignors to The Limited, New York,

N. Y., a corporation of New York Application August '6, 1937, Serial No.157,690

' 7 Claims.

' This invention relates to a process for electrocontaining tin and toan appa- It is more particularly directed to a process and apparatus forelectrolyzing tin tetrachloride. It has heretofore been proposed toelectrolyze solutions of tin tetrachloride, but one literature referencestates that this could not be done satisfactorily. According to thisreference (Mellor VII p. 439) the electrical conductivity of tintetrachloride is .nil' or very small. According to this same source tintetrachloride is not ionized and electrolysis of an aqueous solutionyielded only a small deposit on the cathode due to secondary reactions,since it is improbable that the salt is revolved into Sn"" and 401'ions. other proposal made about thirty years ago was to electrolyzeaqueous solutions of tin tetrachloride in a diaphragm cell for as long aperiod as a commercial current efi'lciency could be maintained. Thebasic anolyte and acid catholyte were then to be withdrawn from theelectric system, combined, concentrated and returned for electrolysis.This process is not a practical one and could not be commerciallypracticed.

An object of the present invention is to provide a simple, practicalprocess for depositing tin, which may be conducted without interruptionif desired and without such reconditioning of the electrolyte. Anotherobject is to provide a simple apparatus for carrying out such I process.Other objects will be in part obvious and in part pointed outhereinafter.

The invention accordingly comprises the novel processes and steps ofprocesses, as well as the novel apparatus and combinations thereof, in

which such processes and steps -may be carried out, specific embodimentsof which are described herein by way of example only and in accordancewith the manner in which we now prefer to practice the invention.

Although the process of our invention may be carried out in other formsof apparatus, .we will now describe an apparatus forming part of ourinvention in which we prefer to operate the process. In the accompanyingdrawings, Fig. 1 represents a sectional elevation on the line |l of Fig.2 of such apparatus, with portions shown diagrammatically; and Fig. 2 isa plan view of the apparatus shown in Fig. 1.

The following description of the apparatus shown in these drawings is adescription of one electrolytic cell with its appurtenant apparatus.Although only one such unit is described it will be clear to thoseskilled in the art that two or any number of such units may be suitablyconnected to form a battery of cells. Such dupli-- cation of units andtheir connections will be well understood to those skilled in the art.

Referring now to the drawings, there is shown an electrolytic cellcomprising a tank I, preferably of concrete, lined on'the interior 2with a Harvel lining, which is prepared from suitably treated oil ofcashew nuts. The Harvel lining composition is made by mixing oil ofcashew nuts with para-formaldehyde. It is adapted to withstand thecorrosive action of the solution 3 being electrolyzed. Other liningshaving a similar resistance may be employed. The electrolyte is suppliedto the head tank 4 and passes therefrom by pipe 5 into the tank I. Thehead tank is part of a system for continuously circulating theelectrolyte 2. This system comprises a sump tank 6 connected by anoverflow pipe 1 to receive electrolyte overflowing through I from thetank I, such overflow electrolyte being delivered into the sump tank 6.The system also includes piping 8 leading back to the head tank 4, inwhich piping a rubber-lined pump 9 is inserted. A heat interchanger Illmay also be inserted in this line, if

desired. This system is made up of or lined with rubber or othermaterial which will withstand the corrosive action of the circulatingelectrolyte.

Located in the electrolytic tank is a series of alternate cathodes II,of soft steel or iron, and anode assemblies. The anode assemblies shownin Fig. 1 are each identical and the description of one will suffice forall. In each of the anode assemblies is an impregnated anode P2 ofgraphite. We prefer to use Atchison graphite. The graphite isimpregnated with boiled linseed oil by immersing. the anodes in the oilat about 220 F. and allowing them to remain there for 60 hours. Insteadof boiled linseed oil, impregnation may be made by similarly immersingthe graphite anodes in melted paraffin. We, however, prefer the linseedoil-impregnated graphite anodes.

Each anode has a cross-bar l3, an end of each resting on bus bar Itresting on the top of the tank 2 for conducting the current into theanode, the opposite end being suitably supported on the opposite wall ofthe tank. i

About each anode is a chamber made up of a wood cap I5 secured theretofrom which depends a bag or envelope l6 which is preferably of glasscloth. The wood cap I5 may be made of hard rubber, if desired. The glasscloth is permeable to the electrolyte and to'the current.-

It also resists, as does the wood where it dips into the electrolytesolution, the corrosive actionof the electrolyte solution. The bags andwooden collars are designed so that they will be disturbed as little aspossible if and when the anodes are changed. The edges of the glassfabric are preferably hemmed to prevent raveling where there is noselvage. It is also preferable to double the top of the bag which fitsover the wooden collar. The solution creeps up into the cloth above thesolution level of the electrolyte and makes the cloth gas-tight.

This bag IB extends somewhat below the anode (as shown in Fig. 1 and isopen at the bottom. Instead of glass cloth we may employ asbestos orother material which will withstand the corrosive action of the solutionbeing electrolyzed and at the same time is permeable to the solution andthe current.

A pipe I! communicates with the chamber about the anode. In this chamberchlorine gas developed at the anode collects and passes then through thepipe I! to a manifold 18 and thence by pipes l9 and 20 to a liquid trap2| where any liquid mechanically entrained with the chlorine gas isseparated, the gas passing out through the top of the trap through apipe 22 to a recovery device. The gas may be compressed to form liquidchlorine ready to be marketed or it may be converted into bleachingpowder by known methods. The pipes l1, I8, I9, 20 and 22, as well as theliquid trap 2|, are all preferably made of rubber or lined with rubberor material capable of resisting chlorine.

The cathodes are composed of strips of soft steel l l as noted and theseare supported by crossbars 23 which have one end resting on bus bars 24supported by the top of the side wall of the tank 2 and the other endrests on the top of the opposite wall of the tank 2.

The top of the tank is closed by a. cover of hard vulcanized rubber orwood 25 vented at 26. The anodes l2 project therethrough and thecathodes II also project therethrough. Each cathode is provided with awooden cap 21 which is attached thereto and which serves to cover theopening in the cover 25 through which the cathode passes into itsposition in the tank 24.

The sump tank 6 is provided with a valved opening 28 through which freshsolution of tin tetrachloride may be poured to replenish that used up bythe electrolysis.

In operating the apparatus, tin tetrachloride to be electrolyzed isplaced in the sump tank 6 and the pump 9 is started and the electrolyteis fed in until a suflicient quantity for electrolysis has been pumped.Anodes and cathodes being in place the current is turned on andelectrolysis commences. Chlorine gas forming at the anode collects inthe cap and bag and chamber, and passes out through the pipe l1, throughthe manifold I 8 and after being freed from liquid, passes out throughthe pipe 22 for recovery. The tin deposits on the cathode II. Theelectrolysis is continued until a sufiicient quantity of tin isdeposited on the cathode which will occur in about 4 or days.

Although not restricted to such use, we may.

. employ the apparatus above described for the carrying out of theprocess of our invention.

We have found in accordance with our invention that aqueous solutionscontaining tin tetrachloride may be electrolyzed with practicalcommercial results. We have found in accordance with our invention thatthe tin deposit should be dense and it is important that after the tinhas been deposited that conditions be maintained such that there issubstantially no re-solution of the tin by the solution beingelectrolyzed. At the same time it is also important that a satisfactorycurrent efilciency shall be maintained.

In carrying out the process of our invention in order to produce a highgrade product and to conduct the process efficiently, we have found itimportant to distill the tin tetrachloride in order to obtain it in apurified form. We have found that the presence of impurities has astrong infiuence on the electrolysis, and if present to any considerableextent will interfere with the operation of electrolysis for an extendedperiod. Various impurities may be found in tin tetrachloride such aslead arsenic, antimony, selenium, tellurium, sulphur, etc. Theseimpurities if not removed tend to deposit on the cathode at thebeginning of the run and form a dark and loosely adhering powder or maypromote the formation of trees. Sulphur is a very undesirable impuritybecause of its destructive effect on the graphite anodes. If not removedit may be necessary to shut down the process and filter the electrolyteto get rid of elementary sulphur. We have found that attempts to removeimpurities by cementation, that is by the addition of metallic tin to anaqueous solution of tin tetrachloride, is not satisfactory. We prefer toemploy a redistillation of tin tetrachloride at a temperature of -120"C., and in this way a tin tetrachloride suitable for electrolysis can beobtained.

In conducting the process an electrolytic tank has been employed likethat described hereinabove, having a length of approximately 12 feet,and a width of about 30 inches and a depth of electrolyte in the tank ofabout 45 inches. The ranges of concentrations of hydrochloric acid andchlorine and tin content in the solution being electrolyzed, as well asthe temperature employed referred to specifically below, have beenascertained by using this apparatus and satisfactory results have beenobtained thereby. It is possible that the limits mentioned may varysomewhat in case of the use of other apparatus of larger or,

smaller size or differently constructed, but such variations or changeswould be clear to those skilled in the art upon operation of such otherapparatus. In preparing to use the apparatus herein described of thedimensions set forth, tin tetrachloride preferably purified as abovedescribed is dissolved in water so that there is present at least 30 to40 grams, but preferably 50-60 grams, of tin per liter. We have foundthat this solution should contain about 45 grams per liter ofhydrochloric acid and this is preferably added to it prior to commencingelectrolysis so that a dense tin deposit shall be obtained. Theexpression dense tin deposit as used herein and in the appended claims,is intended to designate a bright, dense and smooth form of tindeposited on the cathode when the process is carried out in accordancewith our invention. The character of this deposit will naturally varysomewhat as operating conditions vary. It should not contain too large aportion of loose and spongy metal to interfere with the efiicientoperation of the process,

Prior to starting the electrolysis, and thereafter if desired, cresylicacid or similar addition agent is preferably added to the solution. Thecresylic acid is added in the amount of approximately 27 pounds per tonof tin deposited at the cathode. The cresylic acid is added by mixing itwith a,

the electrolysis is cathode and chlorine gas forms, and is collectedabout the anode in a suitable chamber from which it is sucked out.During the course of the electrolysis both chlorine and hydrochloricacid are present in the solution being electrolyzed, and we. have foundthat it is important to maintain a concentration of at least about 45 toabout 120 grams per liter of hydrochloric acid in order to obtain adense deposit. We have also found that it is important to regulate thecontent of chlorine in the solution and to maintain it at about 1.0 gramper liter or less. Although it is important to have suillcienthydrochloric acid present in order to produce a "dense deposit of tin,if the content of hydrochloric acid rises much above 120 grams per literor the content of the chlorine rises much above 1 gram per liter thereis apt to be re-solution of the tin. deposited. It is also important tomaintain a suitable temperature in the solution and we have found thatwith a circulation of about 3 gallons per minute of electrolyte, or atsuch a rate as to prevent the electrolyte leaving the deposition tanksfrom falling below 30gms. per liter of tin, and with a current densityof approximately 30- amperes per square foot cathode area, and with theelectrolytecontent of tin, hydrochloric acid, chlorine and cresylic acidas indicated above, that a temperature of about 130-100 F. will producean efficient electrolysis. This temperature under the conditions givenabove may be maintained without adding any heat to the system, but if itis desired to heat or cool the electrolyte this may be done by applyingor removing heat byheatinterchanger ill, for example, as hereinabovementioned. I f

For each 1 pound of tin deposited at the cathode approximately 1.2pounds of chlorine are liberated at the anode,and itisthereforenecessary (1) that an anode is used which will withstand theeffect of this highly corrosive gas, and (2) that provisionsare made towithdraw the chlorine from the systemeflectively so that it mayberecovered. Since it isimportanttd maintain a low content of chlorine and.since' chlorine is beingv generated at the anode, the chlorine gasshouldbe removed and this is preferably done by collecting the chlorinein a bag such as shown by l6 in Fig. 1 above mentioned and suckme itthrough the system such as shown in Fig. l of the drawings forrecovering chlorine. Most of the chlorine will be removed in this way.Some of it necessarily will evaporate from the surface of thecirculating liquid and hence in the apparatus used it is desirable tovent this chlorine as described. It is important that the anodes It beof a material which will resist chlorine. The anodes described above inconnection with the description of the apparatus are made of a highgrade graphite, preferably impregnated with linseed oil or similarcorrosion-resistant.material.

When a sufilcient deposition of tin is collected on the cathode whichwill occur in about 4 to 5 days, the cathodes are removed and placed ina cathode rack and run through a steam-sweating process which may belike that described in Patent No. 1,989,852.

The following is an example oi. an embodiment of the process as we nowprefer to practice it. It will be understood that this example is purelyillustrative and that the invention is not to be considered as limitedthereto exceptas indicated in the appended claims.

Tin tetrachloride is distilled in a simple vessel constructed of softsteel, this vessel being submerged in an oil bath at a temperature of115-120 C. The tin tetrachloride fumes were condensed in a water-cooledglass coil and collected in a glass vessel to secure a tin tetrachloridecontaining only traces of impurities. In making up the electrolyte, instarting, the purifled tin tetrachloride was dissolved in wateracidified with hydrochloric acid in proportions to give 50-60 grams oftin per liter and approximately 45-50 grams of free hydrochloric acidper liter. Sufllcient crude cresylic acid and glue are then added to thebath of tin'tetrachloride to give a content of 0.5 gram of cresylic acidper liter and 0.1 gram of animal glue per liter.

The solution is now ready for electrolysis and is poured into the sumptank and the pump started and the electrolytic cell -filled to properdepth, and the current being turned on the electrolysis is started. Thetin deposits and chlorine is withdrawn from the solution. After theelectrolysis has been started regular further additions, preferablycontinuous, of the conditioning agents are made at the rate of 27 poundsof cresylic acid and 5.5 pounds of glue for a ton of tin deposited atthe cathode. Additions of tin chloride are made from time to time inorder to keep'th'e tin content at 50-60 grams of tin per liter. Such tintetrachloride solution should 'be redistilled to free it from sulphur,etc. but if it contains precipitated sulphur should be filtered beforeaddition. I

While the electrolysis proceeded care was taken that the temperature didnot drop below F. Such a decrease of temperature increases thesolubility of chlorine gas and, will cause a lower efficiency and ahigher rate of corrosion of the anode. A high chlorine content of theelectrolyte has its destructive effect on the tank lining. Thesedifficulties were avoided by maintaining a temperature of theelectrolyte at about 80- F. The electrolysis was continued for about 220hours with a cathode current density of 30 amperes persquare foot, and acell voltage of 2.3 to 2.9, averaging 2.45. Free' hydrochloric acid'content variedfrom day to day between a 'maxi- ,mum of ,3 grainsperliter'and a'minimum of 72.7 gramsper liter. The over-all currentefliciency noted was'95.3%. The chlorinecontent was less than 1 gram perliter. The tin deposit was then removed by steaming by the process ofPatent No. 1,989,852. The dross produced while remelting amounted toapproximately 1%. The tin recovered upon spectographic investigation hada purity of 99.94%, containing the following small proportions of metalsindicated:

their deposits of tin and replace them by others,

which may-be accomplished in a few minutes.

In the claims the expression oily material referring to the materialemployed for impregnating the carbon anodes is intended to designatelinseed oil as well as parafiin or similar material.

During the operation of the process employing the insoluble anodes,impurities will usually build up in the electrolyte or the amount ofelectrolyte mayhave to be depleted and in that event one of the cells ofthe system should be cut out from the general circulation system and theelectrolytic deposition continued as before, but without addition of tintetrachloride. Cresylic acid and glue are added to the cell and thelatter should be agitated slightly by hand or some mechanical means. Thetin begins to deposit as usual and is gradually removed from thesolution. With a tin content of below grams per liter the cathodedeposit starts to become coarse and rough, this roughness increasingwith the decrease in tin content. The chemical purity of the tin depositis about equal to that of the tin produced during regular operations. Itappears inadvisable to continue the electrolytic depletion of theelectrolyte after about 15 grams of tin per liter has been reached.After this point the depleted electrolyte can be placed in aprecipitation tank where the tin value can be recovered by neutralizingthe tin tetrachloride solution with soda ash or ground limestone toprecipitate the tin as stannic acid which is then resmelted.

While the invention has'been described in detail with respect to aparticular preferred example, it will be understood by those skilled inthe art after understanding the invention, that various changes andfurther modifications may be made without departing from the spirit andscope of the invention, and it is intended therefore in the appendedclaims to cover all such changes and modifications.

What is claimed as new and desired to be secured by Letters Patent ofthe United States solution at a temperature of about F. to F., by addingtin chloride from time to time during the course of the electrolysis andby removing chlorine from the solution, whereby a dense deposit of tinis effected and resolution of the deposited tin is negligible.

2. A process according to claim 1 in which cresylic acid is present inthe solution being electrolyzed.

3. A process according to claim 1 in which cresylic acid and glue arepresent in the solution being electrolyzed.

4. A process as defined in claim 1, in which the electrolysis is carriedout by electrolyzing the solution with a current density of about 30amperes per square foot.

5. A process as defined in claim 1, in which the generated chlorine isremoved from the electrolyte solution'in such amount that the solutionwill not contain more than about 1 gram of chlorine per liter ofsolution.

6. A process of the kind described which comprises, taking forelectrolysis an aqueous solution of tin tetrachloride, substantiallyfree from impurities, adding thereto sumcient hydrochloric acid to givea contentof said acid of at least about 45 grams per liter and not morethan about grams per liter upon the commencement of the electrolysis,electrolyzing the solution in a cell having a chlorine-resistant anodesurrounded by a chamberfor collecting chlorine gas, circulating theelectrolyte through the cell, adding tin tetrachloride to theelectrolyte as electrolysis proceeds and removing chlorine from theelectrolyte so that it contains no more than about 1 gram of chlorineper liter and maintaining the electrolyte solution at a temperature ofabout 80 F. to about 100 F. as well as the other aforesaid conditions ofcirculation, addition of tin tetrachloride and removal of chlorine, sothat the hydrochloric acid content of the electrolyte solution is heldwithin said range of about 45 grams to about 120 grams per liter.

7. An electrolytic process for depositing tincomprising electrolyzing anacidified aqueous solution of tin tetrachloride containing at leastapproximately 30 grams of tin per liter of said solution andapproximately 45 to 120 grams of hydrochloric acid per liter of solutionwithout adding substantially further quantities of tin, to efiectdeposition of the tin and to deplete it below 30 grams per liter but notbelow about 15 grams per liter while forming chlorine and hydrochloricacid, the said concentration of hydrochloric acid in the electrolyticsolution being maintained by maintaining the solution at a temperatureof about 80 F. to 100 F., and by re- Y moving chlorine from thesolution.

